You can sign up for any number of free sandbox sites by visiting here: https://info.recurly.com/signup. To sign up you'll first log out of Recurly before accessing the page above. On that page you'll fill in the details of the form to match you test business needs / or mirror your production site.
As a note, to sign up for a new sandbox you'll need an email address that is NOT associated with any other Recurly sites. Attempting to use an email already in use will not process however, once created you'll have the option to invite any existing user's email address as a site user. After your sandbox is all set up you can go back and add your current login as a user, any other test users and set user permissions.
You can then set up any configurations you have on your current sandbox site by manually re-entering them on your new site. Unfortunately, we do not currently have the functionality to duplicate Recurly sites. There is a pending Feature Request for the ability to Clone a Site/ Components of a Site, so if you're interested in seeing this feature added please reach out to support and we can add your interest to this request.
Our Professional Services Team may also be able to assist in copying your production sites configurations over to your new sandbox. As they have the ability to copy over configurations currently set up such as site settings, plans, coupons, etc. They would not be able to copy any credential based setting such as gateway logins, or 3rd party integrations. If you're interested we can escalate your request to our Professional Services Team to get a quote for this service.